Non-spad pixels for direct time-of-flight range measurement

ABSTRACT

A Direct Time-of-Flight (DTOF) technique is combined with analog amplitude modulation within each pixel in a pixel array. No Single Photon Avalanche Diodes (SPADs) or Avalanche Photo Diodes (APDs) are used. Instead, each pixel has a Photo Diode (PD) with a conversion gain of over 400 μV/e− and Photon Detection Efficiency (PDE) of more than 45%, operating in conjunction with a Pinned Photo Diode (PPD). The TOF information is added to the received light signal by the analog domain-based single-ended to differential converter inside the pixel itself. The output of the PD in a pixel is used to control the operation of the PPD. The charge transfer from the PPD is stopped—and, hence, TOF value and range of an object are recorded—when the output from the PD in the pixel is triggered within a pre-defined time interval. Such pixels provide for an improved autonomous navigation system for drivers.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit under 35 U.S.C. § 119(e) ofU.S. Provisional Application No. 62/607,861 filed on Dec. 19, 2017, thedisclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to image sensors. Morespecifically, and not by way of limitation, particular embodiments ofthe inventive aspects disclosed in the present disclosure are directedto a Time-of-Flight (TOF) image sensor in which a pixel uses a PhotoDiode (PD) with a very high conversion gain to control the operation ofa time-to-charge converter, such as a Pinned Photo Diode (PPD), tofacilitate recording of TOF values and range of a three-dimensional (3D)object.

BACKGROUND

Three-dimensional (3D) imaging systems are increasingly being used in awide variety of applications such as, for example, industrialproduction, video games, computer graphics, robotic surgeries, consumerdisplays, surveillance videos, 3D modeling, real estate sales,autonomous navigation, and so on.

Existing 3D imaging technologies may include, for example, thetime-of-flight (TOF) based range imaging, stereo vision systems, andstructured light (SL) methods.

In the TOF method, distance to a 3D object is resolved based on theknown speed of light—by measuring the round-trip time it takes for alight signal to travel between a camera and the 3D object for each pointof the image. The outputs of pixels in the camera provide informationabout pixel-specific TOF values to generate a 3D depth profile of theobject. A TOF camera may use a scanner-less approach to capture theentire scene with each laser or light pulse. In a direct TOF imager, asingle laser pulse may be used to capture spatial and temporal data torecord a 3D scene. This allows rapid acquisition and rapid real-timeprocessing of scene information. Some example applications of the TOFmethod may include advanced automotive applications such as autonomousnavigation and active pedestrian safety or pre-crash detection based ondistance images in real time, to track movements of humans such asduring interaction with games on video game consoles, in industrialmachine vision to classify objects and help robots find the items suchas items on a conveyor belt, and so on.

Light Detection and Ranging (LiDAR) is an example of a direct TOF methodthat measures distance to a target by illuminating the target with apulsed laser light and measuring the reflected pulses with a sensor.Differences in laser return times and wavelengths can then be used tomake digital 3D representations of the target. LiDAR has terrestrial,airborne, and mobile applications. LiDAR is commonly used to makehigh-resolution maps such as, for example, in archaeology, geography,geology, forestry, and the like. LiDAR also has automotive applicationssuch as, for example, for control and navigation in some autonomouscars.

In stereoscopic imaging or stereo vision systems, two cameras—displacedhorizontally from one another—are used to obtain two differing views ona scene or a 3D object in the scene. By comparing these two images, therelative depth information can be obtained for the 3D object. Stereovision is highly important in fields such as robotics, to extractinformation about the relative position of 3D objects in the vicinity ofautonomous systems/robots. Other applications for robotics includeobject recognition, where stereoscopic depth information allows arobotic system to separate occluding image components, which the robotmay otherwise not be able to distinguish as two separate objects—such asone object in front of another, partially or fully hiding the otherobject. 3D stereo displays are also used in entertainment and automatedsystems.

In the SL approach, the 3D shape of an object may be measured usingprojected light patterns and a camera for imaging. In the SL method, aknown pattern of light—often grids or horizontal bars or patterns ofparallel stripes—is projected onto a scene or a 3D object in the scene.The projected pattern may get deformed or displaced when striking thesurface of the 3D objet. Such deformation may allow an SL vision systemto calculate the depth and surface information of the object. Thus,projecting a narrow band of light onto a 3D surface may produce a lineof illumination that may appear distorted from other perspectives thanthat of the projector, and can be used for geometric reconstruction ofthe illuminated surface shape. The SL-based 3D imaging may be used indifferent applications such as, for example, by a police force tophotograph fingerprints in a 3D scene, inline inspection of componentsduring a production process, in health care for live measurements ofhuman body shapes or the micro structures of human skin, and the like.

SUMMARY

In one embodiment, the present disclosure is directed to a pixel in animage sensor. The pixel comprises: (i) a Photo Diode (PD) unit having atleast one PD that converts received luminance into an electrical signal,wherein the at least one PD has a conversion gain that satisfies athreshold; (ii) an amplifier unit connected in series with the PD unitto amplify the electrical signal and to responsively generate anintermediate output; and (iii) a Time-to-Charge Converter (TCC) unitcoupled to the amplifier unit and receiving the intermediate outputtherefrom. In the pixel, the TCC unit includes: (a) a device that storesan analog charge, and (b) a control circuit coupled to the device. Thecontrol circuit performs operations comprising: (1) initiating transferof a portion of the analog charge from the device, (2) terminating thetransfer in response to receipt of the intermediate output within apre-defined time interval, and (3) generating a pixel-specific outputfor the pixel based on the portion of the analog charge transferred. Inparticular embodiments, the threshold for the conversion gain is atleast 400 μV (microvolts) per photoelectron.

In another embodiment, the present disclosure is directed to a method,which comprises: (i) projecting a laser pulse onto a three-dimensional(3D) object; (ii) applying an analog modulating signal to a device in apixel, wherein the device stores an analog charge; (iii) initiatingtransfer of a portion of the analog charge from the device based onmodulation received from the analog modulating signal; (iv) detecting areturned pulse using the pixel, wherein the returned pulse is theprojected laser pulse reflected from the 3D object, and wherein thepixel includes a Photo Diode (PD) unit having at least one PD thatconverts luminance received in the returned pulse into an electricalsignal and that has a conversion gain that satisfies a threshold; (v)processing the electrical signal using an amplifier unit in the pixel toresponsively generate an intermediate output; (vi) terminating thetransfer of the portion of the analog charge in response to generationof the intermediate output within a pre-defined time interval; and (vii)determining a Time of Flight (TOF) value of the returned pulse based onthe portion of the analog charge transferred upon termination. In someembodiments, the threshold for the conversion gain is at least 400 μVper photon.

In yet another embodiment, the present disclosure is directed to asystem, which comprises: (i) a light source; (ii) a plurality of pixels;(iii) a memory for storing program instructions; and (iv) a processorcoupled to the memory and to the plurality of pixels. In the system, thelight source projects a laser pulse onto a 3D object. In the pluralityof pixels, each pixel includes: (a) a pixel-specific PD unit having atleast one PD that converts luminance received in a returned pulse intoan electrical signal, wherein the at least one PD has a conversion gainthat satisfies a threshold, and wherein the returned pulse results fromreflection of the projected laser pulse by the 3D object; (b) apixel-specific amplifier unit connected in series with thepixel-specific PD unit to amplify the electrical signal and toresponsively generate an intermediate output; and (c) a pixel-specificTCC unit coupled to the pixel-specific amplifier unit and receiving theintermediate output therefrom. In the system, the pixel-specific TCCunit includes: (i) a device that stores an analog charge, and (ii) acontrol circuit coupled to the device. The control circuit performsoperations comprising: (a) initiating transfer of a pixel-specific firstportion of the analog charge from the device; (b) terminating thetransfer of the pixel-specific first portion upon receipt of theintermediate output within a pre-defined time interval; (c) generating afirst pixel-specific output for the pixel based on the pixel-specificfirst portion of the analog charge transferred; (d) transferring apixel-specific second portion of the analog charge from the device,wherein the pixel-specific second portion is substantially equal to aremainder of the analog charge after the pixel-specific first portion istransferred; and (e) generating a second pixel-specific output for thepixel based on the pixel-specific second portion of the analog chargetransferred. In the system, the processor executes the programinstructions, whereby the processor performs the following operationsfor each pixel in the plurality of pixels: (a) facilitating transfers ofthe pixel-specific first and second portions of the analog charge,respectively; (b) receiving the first and the second pixel-specificoutputs; (c) generating a pixel-specific pair of signal values based onthe first and the second pixel-specific outputs, respectively, whereinthe pixel-specific pair of signal values includes a pixel-specific firstsignal value and a pixel-specific second signal value; (d) determining acorresponding pixel-specific TOF value of the returned pulse using thepixel-specific first signal value and the pixel-specific second signalvalue; and (e) determining a pixel-specific distance to the 3D objectbased on the pixel-specific TOF value. In certain embodiments, thethreshold for the conversion gain is at least 400 μV per photoelectron.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following section, the inventive aspects of the presentdisclosure will be described with reference to exemplary embodimentsillustrated in the figures, in which:

FIG. 1 shows a highly simplified, partial layout of a LiDAR TOF imagingsystem according to one embodiment of the present disclosure;

FIG. 2 illustrates an exemplary operational layout of the system in FIG.1 according to one embodiment of the present disclosure;

FIG. 3 depicts exemplary circuit details of a pixel according to certainembodiments of the present disclosure;

FIG. 4 shows exemplary circuit details of another pixel according tosome embodiments of the present disclosure;

FIG. 5 provides circuit details of an exemplary TCC unit in a pixel asper particular embodiments of the present disclosure;

FIG. 6 is an exemplary timing diagram that provides an overview of themodulated charge transfer mechanism in the TCC unit of FIG. 5 accordingto one embodiment of the present disclosure;

FIG. 7 shows the block diagram of an exemplary logic unit that may beused in the TCC unit of FIG. 5 as per particular embodiments of thepresent disclosure;

FIG. 8 is a timing diagram that shows exemplary timing of differentsignals in the system of FIGS. 1-2 when the TCC unit in the embodimentof FIG. 5 is used in a pixel as part of a pixel array for measuring TOFvalues according to certain embodiments of the present disclosure;

FIG. 9 shows circuit details of another exemplary TCC unit as perparticular embodiments of the present disclosure;

FIG. 10 is a timing diagram that shows exemplary timing of differentsignals in the system of FIGS. 1-2 when the TCC unit in the embodimentof FIG. 9 is used in a pixel as part of a pixel array for measuring TOFvalues according to certain embodiments of the present disclosure;

FIG. 11 depicts an exemplary flowchart showing how a TOF value may bedetermined in the system of FIGS. 1-2 according to one embodiment of thepresent disclosure; and

FIG. 12 depicts an overall layout of the system in FIGS. 1-2 accordingto one embodiment of the present disclosure.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the disclosure.However, it will be understood by those skilled in the art that thedisclosed inventive aspects may be practiced without these specificdetails. In other instances, well-known methods, procedures, componentsand circuits have not been described in detail so as not to obscure thepresent disclosure. Additionally, the described inventive aspects can beimplemented to perform low power, range measurements and 3D imaging inany imaging device or system, including, for example, a computer, anautomobile navigation system, and the like.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present disclosure. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” or“according to one embodiment” (or other phrases having similar import)in various places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments. Also, depending on the context of discussionherein, a singular term may include its plural forms and a plural termmay include its singular form. Similarly, a hyphenated term (e.g.,“three-dimensional,” “pre-defined”, “pixel-specific,” etc.) may beoccasionally interchangeably used with its non-hyphenated version (e.g.,“three dimensional,” “predefined”, “pixel specific,” etc.), and acapitalized entry (e.g., “Projector Module,” “Image Sensor,” “PIXOUT” or“Pixout,” etc.) may be interchangeably used with its non-capitalizedversion (e.g., “projector module,” “image sensor,” “pixout,” etc.). Suchoccasional interchangeable uses shall not be considered inconsistentwith each other.

It is noted at the outset that the terms “coupled,” “operativelycoupled,” “connected”, “connecting,” “electrically connected,” etc., maybe used interchangeably herein to generally refer to the condition ofbeing electrically/electronically connected in an operative manner.Similarly, a first entity is considered to be in “communication” with asecond entity (or entities) when the first entity electrically sendsand/or receives (whether through wireline or wireless means) informationsignals (whether containing address, data, or control information)to/from the second entity regardless of the type (analog or digital) ofthose signals. It is further noted that various figures (includingcomponent diagrams) shown and discussed herein are for illustrativepurpose only, and are not drawn to scale. Similarly, various waveformsand timing diagrams are shown for illustrative purpose only.

The terms “first,” “second,” etc., as used herein, are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.) unless explicitly defined assuch. Furthermore, the same reference numerals may be used across two ormore figures to refer to parts, components, blocks, circuits, units, ormodules having the same or similar functionality. However, such usage isfor simplicity of illustration and ease of discussion only; it does notimply that the construction or architectural details of such componentsor units are the same across all embodiments or such commonly-referencedparts/modules are the only way to implement the teachings of particularembodiments of the present disclosure.

It is observed here that the earlier-mentioned 3D technologies have manydrawbacks. For example, a range gated TOF imager may use multiple laserpulses to provide illumination and an optical gate to allow the light toreach the imager only during a desired time period. The range gated TOFimagers can be used in two-dimensional (2D) imaging to suppress anythingoutside a specified distance range, such as to see through fog. However,the gated TOF imagers may provide only Black-and-White (B&W) output andmay not have 3D imaging capability. Furthermore, current TOF systemstypically operate over a range of few meters to several tens of meters,but their resolution may decrease for measurements over short distances,thereby making 3D imaging within a short distance—such as, for example,in a fog or hard-to-see conditions—almost impractical. Also, pixels inexisting TOF sensors may be vulnerable to ambient light.

Direct TOF (DTOF) LiDAR sensors typically use Single Photon AvalancheDiodes (SPADs) or Avalanche Photo Diodes (APDs) in their pixel arraysfor DTOF range measurements. Generally, SPAD and APD both require a highoperating voltage—in the range of approximately 20V to 30V—and specialfabrication processes to manufacture them. Furthermore, a SPAD has a lowPhoton Detection Efficiency (PDE) in the range of 5%. Thus, a SPAD basedimager may not be optimum for a high speed 3D imaging system forall-weather autonomous navigation.

The stereoscopic imaging approach generally works only with texturedsurfaces. It has high computational complexity because of the need tomatch features and find correspondences between the stereo pair ofimages of an object. This requires high system power. Furthermore,stereo imaging requires two regular, high bit resolution sensors alongwith two lenses, making the entire assembly unsuitable where space is ata premium such as, for example, in an automobile-based autonomousnavigation system. Additionally, stereo 3D cameras have difficulty tosee through fog and to deal with motion blur.

In contrast, particular embodiments of the present disclosure providefor implementing a lost cost, high performance automotive LiDAR sensoror DTOF-based 3D imaging system on automotives for all weatherconditions. Thus, improved vision for drivers may be provided underdifficult conditions such as, for example, low light, bad weather, fog,strong ambient light, and the like. A DTOF range measurement system asper particular embodiments of the present disclosure may not includeimaging, but, instead, may provide an audible and/or a visible alert.The measured range may be used in autonomous control of a vehicle suchas, for example, automatically stopping a vehicle to avoid collisionwith another object. As discussed in more detail below, in a singlepulse-based direct TOF system as per particular embodiments of thepresent disclosure, the TOF information is added to the received signalby means of controlled charge transferring and analog domain-basedsingle-ended to differential converter inside the pixel itself. Thus,the present disclosure provides for a single chip solution that directlycombines TOF and analog Amplitude Modulation (AM) within each pixel inthe pixel array using a high conversion Photo Diode (PD)-having PDE inthe range of 45% or more—in conjunction with a single Pinned Photo Diode(PPD) (or another time-to-charge converter) in each pixel. The highconversion PDs replace the SPADs in the current LiDAR imagers for DTOFrange measurements. The output of the PD in a pixel is used to controlthe operation of the PPD to facilitate recording of TOF values and rangeof a 3D object. As a result, an improved autonomous navigation systemmay be offered that can “see through” inclement weather at short rangeand produce 3D images as well as 2D gray-scale images under asubstantially lower operating voltage.

FIG. 1 shows a highly simplified, partial layout of a LiDAR TOF imagingsystem 15 according to one embodiment of the present disclosure. Asshown, the system 15 may include an imaging module 17 coupled to and incommunication with a processor or host 19. The system 15 may alsoinclude a memory module 20 coupled to the processor 19 to storeinformation content such as, for example, image data received from theimaging module 17. In particular embodiments, the entire system 15 maybe encapsulated in a single Integrated Circuit (IC) or chip.Alternatively, each of the modules 17, 19, and 20 may be implemented ina separate chip. Furthermore, the memory module 20 may include more thanone memory chip, and the processor module 19 may comprise of multipleprocessing chips as well. In any event, the details about packaging ofthe modules in FIG. 1 and how they are fabricated or implemented—in asingle chip or using multiple discrete chips—are not relevant to thepresent discussion and, hence, such details are not provided herein.

The system 15 may be any electronic device configured for 2D and 3Dimaging applications as per teachings of the present disclosure. Thesystem 15 may be portable or non-portable. Some examples of the portableversion of the system 15 may include popular consumer electronic gadgetssuch as, for example, a mobile device, a cellphone, a smartphone, a UserEquipment (UE), a tablet, a digital camera, a laptop or desktopcomputer, an automobile navigation unit, a Machine-to-Machine (M2M)communication unit, a Virtual Reality (VR) equipment or module, a robot,and the like. On the other hand, some examples of the non-portableversion of the system 15 may include a game console in a video arcade,an interactive video terminal, an automobile with autonomous navigationcapability, a machine vision system, an industrial robot, a VRequipment, and so on. The 3D imaging functionality provided as perteachings of the present disclosure may be used in many applicationssuch as, for example, automobile applications such as all-weatherautonomous navigation and driver assistance in low light or inclementweather conditions, human-machine interface and gaming applications,machine vision and robotics applications, and the like.

In particular embodiments of the present disclosure, the imaging module17 may include a projector module (or light source module) 22 and animage sensor unit 24. As discussed in more detail with reference to FIG.2 below, in one embodiment, the light source in the projector module 22may be an Infrared (IR) laser such as, for example, a Near Infrared(NIR) or a Short Wave Infrared (SWIR) laser, to make the illuminationunobtrusive. In other embodiments, the light source may be a visiblelight laser. The image sensor unit 24 may include a pixel array andancillary processing circuits as shown in FIG. 2 and also discussedbelow.

In one embodiment, the processor 19 may be a Central Processing Unit(CPU), which can be a general purpose microprocessor. In the discussionherein, the terms “processor” and “CPU” may be used interchangeably forease of discussion. However, it is understood that, instead of or inaddition to the CPU, the processor 19 may contain any other type ofprocessors such as, for example, a microcontroller, a Digital SignalProcessor (DSP), a Graphics Processing Unit (GPU), a dedicatedApplication Specific Integrated Circuit (ASIC) processor, and the like.Furthermore, in one embodiment, the processor/host 19 may include morethan one CPU, which may be operative in a distributed processingenvironment. The processor 19 may be configured to execute instructionsand to process data according to a particular Instruction SetArchitecture (ISA) such as, for example, an x86 instruction setarchitecture (32-bit or 64-bit versions), a PowerPC® ISA, or a MIPS(Microprocessor without Interlocked Pipeline Stages) instruction setarchitecture relying on RISC (Reduced Instruction Set Computer) ISA. Inone embodiment, the processor 19 may be a System on Chip (SoC) havingfunctionalities in addition to a CPU functionality.

In particular embodiments, the memory module 20 may be a Dynamic RandomAccess Memory (DRAM) such as, for example, a Synchronous DRAM (SDRAM),or a DRAM-based Three Dimensional Stack (3DS) memory module such as, forexample, a High Bandwidth Memory (HBM) module, or a Hybrid Memory Cube(HMC) memory module. In other embodiments, the memory module 20 may be aSolid State Drive (SSD), a non-3DS DRAM module, or any othersemiconductor-based storage system such as, for example, a Static RandomAccess Memory (SRAM), a Phase-Change Random Access Memory (PRAM orPCRAM), a Resistive Random Access Memory (RRAM or ReRAM), aConductive-Bridging RAM (CBRAM), a Magnetic RAM (MRAM), a Spin-TransferTorque MRAM (STT-MRAM), and the like.

FIG. 2 illustrates an exemplary operational layout of the system 15 inFIG. 1 according to one embodiment of the present disclosure. The system15 may be used to obtain range measurements (and, consequently, a 3Dimage) for a 3D object, such as the 3D object 26, which may be anindividual object or an object within a group of other objects. In oneembodiment, the range and 3D depth information may be calculated by theprocessor 19 based on the measurement data received from the imagesensor unit 24. In another embodiment, the range/depth information maybe calculated by the image sensor unit 24 itself. In particularembodiments, the range information may be used by the processor 19 aspart of a 3D user interface to enable the user of the system 15 tointeract with the 3D image of the object or use the 3D image of theobject as part of games or other applications—like an autonomousnavigation application—running on the system 15. The 3D imaging as perteachings of the present disclosure may be used for other purposes orapplications as well, and may be applied to substantially any 3D object,whether stationary or in motion.

The light source (or projector) module 22 may illuminate the 3D object26 by projecting a short pulse 28 as shown by an exemplary arrow 30associated with a corresponding dotted line 31 representing anillumination path of a light signal or optical radiation that may beused to project on the 3D object 26 within an optical Field Of View(FOV). The system 15 may be a direct TOF imager in which a single pulsemay be used per image frame (of pixel array). In certain embodiments,multiple, short pulses may be transmitted onto the 3D object 26 as well.An optical radiation source, which, in one embodiment, may be a laserlight source 33 operated and controlled by a laser controller 34, may beused to project the short pulse 28 (here, a laser pulse) onto the 3Dobject 26. The short pulse 28 from the laser light source 33 may beprojected—under the control of the laser controller 34—onto the surfaceof the 3D object 26 via projection optics 35. The projection optics maybe a focusing lens, a glass/plastics surface, or other cylindricaloptical element. In the embodiment of FIG. 2, a convex structure, suchas a focusing lens, is shown as projection optics 35. However, any othersuitable lens design or an external optical cover may be selected forprojection optics 35.

In particular embodiments, the light source (or illumination source) 33may be a diode laser or a Light Emitting Diode (LED) emitting visiblelight, a light source that produces light in the non-visible spectrum,an IR laser (for example, an NIR or an SWIR laser), a point lightsource, a monochromatic illumination source (such as, for example, acombination of a white lamp and a monochromator) in the visible lightspectrum, or any other type of laser light source. In autonomousnavigation applications, the more unobtrusive NIR or SWIR laser may bepreferred as the pulsed laser light source 33. In certain embodiments,the laser light source 33 may be one of many different types of laserlight sources such as, for example, a point source with 2D scanningcapability, a sheet source with one-dimensional (1D) scanningcapability, or a diffused laser with matching FOV of the image sensorunit 24. In particular embodiments, the laser light source 33 may befixed in one position within the housing of the device 15, but may berotatable in X-Y directions. The laser light source 33 may be X-Yaddressable (for example, by the laser controller 34) to perform a scanof the 3D object 26. The laser pulse 28 may be projected onto thesurface of the 3D object 26 using a mirror (not shown), or theprojection may be completely mirror-less. In particular embodiments, theprojector module 22 may include more or less components than those shownin the exemplary embodiment of FIG. 2.

In the embodiment of FIG. 2, the light/pulse 37—also referred to as the“returned pulse”—reflected from the object 26 may travel along acollection path indicated by an arrow 39 adjacent to a dotted line 40.The light collection path may carry photons reflected from or scatteredby the surface of the object 26 upon receiving illumination from thelaser source 33. It is noted here that the depiction of variouspropagation paths using solid arrows and dotted lines in FIG. 2 is forillustrative purpose only. The depiction should not be construed toillustrate any actual optical signal propagation paths. In practice, theillumination and collection signal paths may be different from thoseshown in FIG. 2, and may not be as clearly-defined as in theillustration in FIG. 2.

In TOF imaging, the light received from the illuminated 3D object 26 maybe focused onto a 2D pixel array 42 via collection optics 44 in theimage sensor unit 24. The pixel array 42 may include one or more pixels43. Like the projection optics 35, the collection optics 44 may be afocusing lens, a glass/plastics surface, or other cylindrical opticalelement that concentrates the reflected light received from the 3Dobject 26 onto one or more pixels 43 in the 2D array 42. An opticalband-pass filter (not shown) may be used as part of the collectionoptics 44 to pass only the light with the same wavelength as thewavelength of light in the laser pulse 28. This may help suppresscollection/reception of non-relevant light and reduce noise. In theembodiment of FIG. 2, a convex structure, such as a focusing lens, isshown as the collection optics 44. However, any other suitable lensdesign or optical covering may be selected for collection optics 44.Furthermore, for ease of illustration, only a 3×3 pixel array is shownin FIG. 2. However, it is understood that, modern pixel arrays containthousands or even millions of pixels.

The TOF-based 3D imaging as per particular embodiments of the presentdisclosure may be performed using many different combinations of 2Dpixel arrays 42 and laser light sources 33 such as, for example: (i) a2D color (RGB) sensor with a visible light laser source, in which thelaser source may be a red (R), green (G), or blue (B) light laser, or alaser source producing a combination of these lights; (ii) a visiblelight laser with a 2D RGB color sensor having an Infrared (IR) cutfilter; (iii) an NIR or SWIR laser with a 2D IR sensor; (iv) an NIRlaser with a 2D NIR sensor; (v) an NIR laser with a 2D RGB sensor(without an IR cut filter); (vi) an NIR laser with a 2D RGB sensor(without an NIR cut filter); (vii) a 2D RGB-IR sensor with visible or IRlaser; (viii) a 2D RGBW (red, green, blue, white) or RWB (red, white,blue) sensor with either visible or NIR laser; and so on. In case of anNIR or other IR laser as, for example, in autonomous navigationapplications, the 2D pixel array 42 may provide outputs to generate agrayscale image of the 3D object 26. These pixel outputs also may beprocessed to obtain the range measurements and, hence, to generate a 3Dimage of the object 26, as discussed in more detail below. Exemplarycircuit details of individual pixels 43 are shown and discussed laterwith reference to FIGS. 3-5, 7, and 9.

The pixel array 42 may convert the received photons into correspondingelectrical signals, which are then processed by the associated imageprocessing unit 46 to determine the range and 3D depth image of theobject 26. In one embodiment, the image processing unit 46 and/or theprocessor 19 may carry out range measurements. As noted in FIG. 2, theimage processing unit 46 may also include relevant processing circuitsand circuits for controlling the operation of the pixel array 42. It isnoted here that both the projector module 22 and the pixel array 42 mayhave to be controlled by high speed signals and synchronized. Thesesignals have to be very accurate to obtain a high resolution. Hence, theprocessor 19 and the image processing unit 46 may be configured toprovide relevant signals with accurate timing and high precision.

In the TOF system 15 in the embodiment of FIG. 2, the image processingunit 46 may receive a pair of pixel-specific outputs from each pixel 43to measure the pixel-specific time (pixel-specific TOF value) the lighthas taken to travel from the projector module 22 to the object 26 andback to the pixel array 42. The timing calculation may use the approachdiscussed below. Based on the calculated TOF values, in certainembodiments, the pixel-specific distance to the object 26 may becalculated by the image processing unit 46 directly in the image sensorunit 24 to enable the processor 19 to provide a 3D distance image of theobject 26 over some interface—such as, for example, a display screen oruser interface.

The processor 19 may control the operations of the projector module 22and the image sensor unit 24. Upon user input or automatically (as, forexample, in a real-time autonomous navigation application), theprocessor 19 may repeatedly send a laser pulse 28 onto the surrounding3D object(s) 26 and trigger the sensor unit 24 to receive and processincoming returned pulses 37. The processed image data received from theimage processing unit 46 may be stored by the processor 19 in the memory20 for TOF-based range computation and 3D image generation (ifapplicable). The processor 19 may also display a 2D image (for example,a grayscale image) and/or a 3D image on a display screen (not shown) ofthe device 15. The processor 19 may be programmed in software orfirmware to carry out various processing tasks described herein.Alternatively or additionally, the processor 19 may compriseprogrammable hardware logic circuits for carrying out some or all of itsfunctions. In particular embodiments, the memory 20 may store programcode, look-up tables, and/or interim computational results to enable theprocessor 19 to carry out its functions.

FIG. 3 depicts exemplary circuit details of a pixel 50 according tocertain embodiments of the present disclosure. The pixel 50 is anexample of the pixel 43 in the pixel array 42 of FIG. 2. For TOFmeasurements, the pixel 50 may operate as a time-resolving sensor, asdiscussed later with reference to FIGS. 5-10. As shown in FIG. 3, thepixel 50 may include a Photo Diode (PD) unit 52 electrically connectedto an output unit 53. The PD unit 52 may include a first PD 55 connectedin parallel with a second PD 56. The first PD 55 may be a very highconversion gain PD operable to convert received luminance (or incominglight)—illustrated by a line with the reference numeral “57”—into anelectrical signal, which may be provided to the output unit 53 via afirst PD-specific output terminal 58 for further processing. In someembodiments, the received luminance 57 may be the luminance received inthe returned pulse 37 (FIG. 2). In particular embodiments, theconversion gain of the first PD 55 may be at least 400 μV perphotoelectron (or photon), which also may be interchangeably referred as400 μV/e−. As mentioned earlier, conventional PDs have conversion gainlower than 200 μV/e−. The high gain PD 55 also may have a much higherPDE—in the range of 45% or more, thereby facilitating photon detectionin low light conditions as well. The PD 55 may perform photon countingwithout avalanche gain and, hence, can be used to replace a SPAD in DTOFLiDAR sensors. Furthermore, the PD 55 may be compatible with other lowvoltage Complementary Metal Oxide Semiconductor (CMOS) circuits and mayoperate at “conventional” supply voltage of around 2.5V to 3V, therebyproviding significant power savings. In contrast, as mentioned before, aSPAD (or an APD) may require a high operating voltage of around 20V to30V. Thus, the pixel 50 comprising the PD 55 with high conversion gain,high PDE, and low operating voltage may be advantageously used in apixel array, such as the pixel array 42 in FIG. 2, in a high speed 3Dimaging system—such as, for example, the system 15 in FIGS. 1-2—forall-weather autonomous navigation and other applications requiringTOF-based range measurements.

In one embodiment, the second PD 56 may be similar to the first PD 55 inthe sense that the second PD 56 also may be a low-voltage PD with a veryhigh gain and high PDE. However, in contrast to the first PD 55, thesecond PD 56 may not be exposed to light—as illustrated by a grey circlearound the PD 56 in FIG. 3. Thus, the second PD 56 may detect the levelof darkness—for example, at the time of reception of luminance 57—andgenerate a reference signal (or dark current) representing the darknesslevel. The reference signal may be provided to the output unit 53 via asecond PD-specific output terminal 59. It is noted that, although onlyone high gain PD 55 is shown in the PD unit 52 as a light receptor, insome embodiments, the PD unit 52 may include more than one PDs similarto the PD 55; all such high gain PDs may be connected in parallel witheach other (and with the unexposed PD 56) and exposed to received light.

It is noted here that, simply for ease of discussion and depending onthe context, the same reference numeral may be used in the discussion ofFIGS. 3-10 to occasionally interchangeably refer to a line/terminal andthe signal associated with that line/terminal. For example, thereference numeral “58” may be used to interchangeably refer to theelectrical signal generated by the PD 55 and the line/terminal carryingthe electrical signal. Similarly, the reference numeral “59” may be usedto refer to the reference signal generated by the PD 56 and theline/terminal carrying the reference signal, the reference numeral “74”(discussed later below) may be used to refer to the electrical signaloutput by the PD unit 68 (FIG. 4) and the line/terminal carrying theelectrical signal, and so on.

An amplifier unit 60 in the output unit 53 may be connected in serieswith the PDs 55-56, and may be operable to amplify the electrical signal58. In some embodiments, the amplifier unit 60 may be a sense amplifier.Prior to such amplification, the sense amplifier 60 may reset the PDs55-56. Thereafter, the PD 55 may receive the luminance 57 and generatethe electrical signal 58. The sense amp 60 may operate to amplify theelectrical signal only when an electronic shutter is turned on.Exemplary shutter signals are shown in FIGS. 6, 8, and 10, which arediscussed later. In the embodiment of FIG. 3, a shutter signal (alsoreferred to as an “electronic shutter”) 61 is shown as anexternally-supplied “Enable” (En) input to the sense amplifier 60. Inone embodiment, the PDs 55-56 may be reset before the shutter signal 61is turned on. While the shutter signal 61 is active, the sense amp 60may sense the electrical signal 58 (generated in response to detectionof photon arrival) vis-à-vis the reference signal (or dark current) 59and amplify the electrical signal to generate an intermediate output 62.In one embodiment, the sense amp 60 may be a conventional current senseamplifier. The intermediate output 62 may be a voltage signal or acurrent signal, depending on implementation.

Exemplary circuit details for the Time-to-Charge Converter (TCC) unit 64are shown in FIGS. 5, 7, and 9, discussed later below. The TCC unit 64may be used to record the photon arrival time based on analog chargetransfer (discussed later). Generally, in particular embodiments, theTCC unit 64 may include a pixel-specific device—such as a Pinned PhotoDiode (PPD) or a capacitor—operable to store an analog charge, and acontrol circuit coupled to the device and operable to: (i) initiatetransfer of a portion of the analog charge from the device, (ii)terminate the transfer in response to receipt of the intermediate output62 within a pre-defined time interval, and (iii) generate apixel-specific analog output (PIXOUT) 65 for the pixel based on theportion of the analog charge transferred. In the embodiment of FIG. 2,the pixout signals from various pixels 43 (similar to the pixel 50 inFIG. 3) in the image sensor array 42 may be processed by the imageprocessing unit 46 (or the processor 19) to record the photon arrivaltime(s) and determine TOF values. Thus, as discussed in more detaillater, the intermediate output 62 (and, hence, the photon detection bythe PD 55) may control the charge transfer from the analog storagedevice (for example, a PPD or a capacitor) to generate thepixel-specific output (Pixout) 65. As also discussed later, the chargetransfer may facilitate recording of a TOF value and corresponding rangeof the 3D object 26. In other words, the output from the PD 55 is usedto determine the operation of the storage device. Furthermore, in thepixel 50, the light-sensing functionality is performed by the PD 55,whereas the analog storage device is used as a time-to-charge converterinstead of a light-sensing element.

FIG. 4 shows exemplary circuit details of another pixel 67 according tosome embodiments of the present disclosure. The pixel 67 is anotherexample of the pixel 43 in the pixel array 42 of FIG. 2. Like the pixel50 in FIG. 3, the pixel 67 also may operate as a time-resolving sensorfor TOF measurements, as discussed later with reference to FIGS. 5-10.As shown in FIG. 4, the pixel 50 may include a Photo Diode (PD) unit 68electrically connected to an output unit 69. In the embodiment of FIG.4, the PD unit 68 may include only one PD 70 with a very high conversiongain and high PDE; an unexposed PD, like the PD 56, may not be includedas part of the PD unit 68. The PD 70, however, may be substantiallysimilar to the PD 55 (FIG. 3) and, hence, earlier discussion of gain,operating voltage, and PDE of the PD 55 applies to the PD 70 as well.Therefore, such earlier discussion is not repeated here for the sake ofbrevity. It is noted that, although only one high gain PD 70 is shown inthe PD unit 68 as a light receptor, in some embodiments, the PD unit 68may include more than one PDs similar to the PD 70; all such high gainPDs may be connected in parallel with each other and exposed to receivedlight.

As shown in FIG. 4, the PD 70 may be operable to receive the incominglight/luminance 71 and may be connected to the generic supply voltageVDD (which may be in the range of 2.5 volts to 3 volts) via a switch 73.As before, the incoming light 71 may represent the luminance received inthe returned pulse 37 (FIG. 2). The PD unit 68 may include a couplingcapacitor 72 through which the electrical signal generated by the PD 70upon detection of one or more photons in the received luminance 71 maybe provided to the output unit 69 via the line/terminal 74. In theembodiment of FIG. 4, a gainstage circuit in the output unit 69 may beused as an amplifier unit to amplify the electrical signal 74. In theembodiment of FIG. 4, the gainstage circuit may include an invertingamplifier (or diode inverter) 75 in parallel with a bypass capacitor 76,as shown. In other embodiments, a non-inverting amplifier may be usedinstead, depending on the subsequent signal processing. A switch 77 maybe provided to reset the gainstage prior to amplification of theelectrical signal 74. The switches 73 and 77 may be controlled by anexternally-supplied shutter signal, such as the electronic shuttersignal 61 mentioned in the context of FIG. 3 earlier. Exemplary shuttersignals are shown in FIGS. 6, 8, and 10, which are discussed later. Whenthe shutter signal 61 is off (or not turned on), the switches 73, 77 mayremain closed, thereby resetting the PD 70 and the gainstage. Thegainstage may operate to amplify the electrical signal 74 only when theelectronic shutter 61 is turned on. When the shutter signal 61 is turnedon (or active), the switches 73, 77 are opened. If the PD 70 receivesthe luminance 71 and generates the electrical signal 74 while theshutter 61 is active, the gainstage may amplify the electrical signal 74to generate an intermediate output 78. The intermediate output 78 may bea voltage signal or a current signal, depending on implementation.

Exemplary circuit details for the TCC unit 79 are shown in FIGS. 5, 7,and 9, discussed later below. Like the TCC unit 64 in FIG. 3, the TCCunit 79 in FIG. 4 also may be used to record the photon arrival timebased on analog charge transfer. In certain embodiments, the TCC units64 and 79 may be identical in construction. Generally, in particularembodiments, the TCC unit 79 may include a pixel-specific device—such asa PPD or a capacitor—operable to store an analog charge, and a controlcircuit coupled to the device and operable to: (i) initiate transfer ofa portion of the analog charge from the device, (ii) terminate thetransfer in response to receipt of the intermediate output 78 within apre-defined time interval, and (iii) generate a pixel-specific analogoutput (PIXOUT) 80 for the pixel based on the portion of the analogcharge transferred. In the embodiment of FIG. 2, the pixout signals fromvarious pixels 43 (similar to the pixel 67 in FIG. 4) in the imagesensor array 42 may be processed by the image processing unit 46 (or theprocessor 19) to record the photon arrival time(s) and determine TOFvalues. Thus, as discussed in more detail later, the intermediate output78 (and, hence, the photon detection by the PD 70) may control thecharge transfer from the analog storage device (for example, a PPD or acapacitor) to generate the pixel-specific output (Pixout) 80. As alsodiscussed later, the charge transfer may facilitate recording of a TOFvalue and corresponding range of the 3D object 26. In other words, theoutput from the high gain PD 70 is used to determine the operation ofthe analog storage device. Furthermore, in the pixel 67, thelight-sensing functionality is performed by the PD 70, whereas theanalog storage device is used as a time-to-charge converter instead of alight-sensing element.

FIG. 5 provides circuit details of an exemplary TCC unit 84 in a pixelas per particular embodiments of the present disclosure. The pixel maybe any of the pixels 50 or 67 (which are examples of the more genericpixel 43 in FIG. 2), and the TCC unit 84 may be any of the TCC units 64or 79. An electronic shutter signal, such as the shutter signal 61 inFIGS. 3-4, may be provided to each pixel (as discussed in more detaillater with reference to the timing diagrams in FIGS. 6, 8, and 10) toenable the pixel to capture the pixel-specific photoelectrons in thereceived light. More generally, the TCC unit 84 may be considered tohave a charge transfer trigger portion, a charge generation and transferportion, and a charge collection and output portion. The charge transfertrigger portion may include a logic unit 86 that receives the signal 87from the relevant amplifier unit—the sense amplifier 60 in case of thepixel 50 in FIG. 3 or the gainstage in case of the pixel 67 in FIG. 4.The signal 87 may represent either of the intermediate outputs 62 and78, as applicable. Block diagram of an exemplary logic unit, such as thelogic unit 86, is shown in FIG. 7, which is discussed later. The chargegeneration and transfer portion may include a PPD 89, a first N-channelMetal Oxide Semiconductor Field Effect Transistor (NMOSFET or NMOStransistor) 90, a second NMOS transistor 91, and a third NMOS transistor92. The charge collection and output portion may include the third NMOStransistor 92, a fourth NMOS transistor 93, and a fifth NMOS transistor94. It is noted here that, in some embodiments, the TCC unit 84 in FIG.5 and the TCC unit 140 in FIG. 9 (discussed later) may be formed ofP-channel Metal Oxide Semiconductor Field Effect Transistors (PMOSFETsor PMOS transistors) or other different types of transistors or chargetransfer devices. Furthermore, the above-mentioned separation of variouscircuit components into respective portions is for illustrative anddiscussion purpose only. In certain embodiments, such portions mayinclude more or less or different circuit elements than those listedhere.

The PPD 89 may store analog charge similar to a capacitor. In oneembodiment, the PPD 89 may be covered and does not respond to light.Thus, the PPD 89 may be used as a time-to-charge converter instead of alight sensing element. However, as noted before, the light-sensingfunctionality may be accomplished through the high gain PD 55 or 70. Incertain embodiments, a photogate, a capacitor, or other semiconductordevice—with suitable circuit modifications—may be used as a chargestorage device instead of a PPD in the TCC units of FIGS. 5 and 9.

Under the operative control of the electronic Shutter signal 61, thecharge transfer trigger portion—such as the logic unit 86—may generate aTransfer Enable (TXEN) signal 96 to trigger the transfer of chargestored in the PPD 89. A PD 55, 70 may detect a photon (which may bereferred to as a “photon detection event”) in the light pulse that wastransmitted and reflected off of an object, such as the object 26 inFIG. 2, and output the electrical signal 87, which may be latched by thelogic unit 86, which may include logic circuits to process theelectrical signal 87 to generate the TXEN signal 96 as discussed laterin the context of FIG. 7.

In the charge generation and transfer portion, the PPD 89 may beinitially set to its full well capacity using a Reset (RST) signal 98 inconjunction with the third transistor 92. The first transistor 90 mayreceive a Transfer Voltage (VTX) signal 99 at its drain terminal and theTXEN signal 96 at its gate terminal. A TX signal 100 may be available atthe source terminal of the first transistor 90 and applied to the gateterminal of the second transistor 91. As shown, the source terminal ofthe first transistor 90 may be connected to the gate terminal of thesecond transistor 91. As discussed later below, the VTX signal 99 (or,equivalently, the TX signal 100) may be used as an analog modulatingsignal to control the analog charge to be transferred from the PPD 89,which may be connected to the source terminal of the transistor 91 inthe configuration shown. The second transistor 91 may transfer thecharge on the PPD 89 from its source terminal to its drain terminal,which may connect to the gate terminal of the fourth transistor 93 andform a charge “collection site” referred to as a Floating Diffusion (FD)node/junction 102. In particular embodiments, the charge transferredfrom the PPD 89 may depend on the modulation provided by the analogmodulating signal 99 (or, equivalently, the TX signal 100). In theembodiments of FIGS. 5 and 10, the charge transferred is electrons.However, the present disclosure is not limited thereto. In anembodiment, a PPD with different design may be used, where the chargetransferred may be holes.

In the charge collection and output portion, the third transistor 92 mayreceive the RST signal 98 at its gate terminal and a Pixel Voltage(VPIX) signal 104 at its drain terminal. The source terminal of thethird transistor 92 may be connected to the FD node 102. In oneembodiment, the voltage level of the VPIX signal 104 may equal to thevoltage level of the generic supply voltage VDD and may be in the rangeof 2.5V (volts) to 3V. The drain terminal of the fourth transistor 93also may receive the VPIX signal 104 as shown. In particularembodiments, the fourth transistor 93 may operate as an NMOS sourcefollower to function as a buffer amplifier. The source terminal of thefourth transistor 93 may be connected to the drain terminal of the fifthtransistor 94, which may be in cascode with the source follower 93 andreceiving a Select (SEL) signal 105 at its gate terminal. The chargetransferred from the PPD 89 and “collected” at the FD node 102 mayappear as the pixel-specific output PIXOUT 107 at the source terminal ofthe fifth transistor 94. The Pixout line/terminal 107 may representeither of the Pixout lines 65 (FIG. 3) or 80 (FIG. 4).

Briefly, as mentioned before, the charge transferred from the PPD 89 toFD 102 is controlled by the VTX signal 99 (and, hence, the TX signal100). The amount of charge reaching the FD node 102 is modulated by theTX signal 100. In one embodiment, the voltage VTX 99 (and, also TX 100)may be ramped to gradually transfer charge from the PPD 89 to FD 102.Thus, the amount of charge transferred may be a function of the analogmodulating voltage TX 100, and the ramping of the TX voltage 100 is afunction of time. Hence, the charge transferred from the PPD 89 to theFD node 102 also is a function of time. If, during the transfer ofcharge from the PPD 89 to FD 102, the second transistor 91 is turned off(for example, becomes open-circuited) due to the generation of the TXENsignal 96 by the logic unit 86 upon a photon detection event for the PD55 (or 70), the transfer of charge from the PPD 89 to the FD node 102stops. Consequently, the amount of charge transferred to FD 102 and theamount of charge remaining in the PPD 89 are both a function of the TOFof the incoming photon(s). The result is a time-to-charge conversion anda single-ended to differential signal conversion. The PPD 89 thusoperates as a time-to-charge converter. The more the charge istransferred to the FD node 102, the more the voltage decreases on the FDnode 102 and the more the voltage increases on the PPD 89. It isobserved that the farther the object 26 (FIG. 2), the more the chargewill be transferred to the FD node 102.

The voltage at the floating diffusion 102 may be later transferred asthe Pixout signal 107 to an Analog-to-Digital Converter (ADC) unit (notshown) using the transistor 94 and converted into an appropriate digitalsignal/value for subsequent processing. More details of the timing andoperation of various signals in FIG. 5 are provided below with referenceto discussion of FIG. 8. In the embodiment of FIG. 5, the fifthtransistor 94 may receive the SEL signal 105 for selecting thecorresponding pixel 50 (or 67) to readout the charge in the floatingdiffusion (FD) 102 as a PIXOUT1 (or Pixel Output 1) voltage and theremaining charge in the PPD 89 as a PIXOUT2 (or Pixel Output 2) voltageafter it is completely transferred to the FD node 102, wherein the FDnode 102 converts a charge on it to a voltage and the pixel output line(PIXOUT) 107 sequentially outputs PIXOUT1 and PIXOUT2 signals asdiscussed later with reference to FIG. 8. In another embodiment, eitherthe PIXOUT1 signal or the PIXOUT2 signal (but not both) may be read out.

In one embodiment, the ratio of one pixel output (for example, PIXOUT1)to the sum of the two pixel outputs (here, PIXOUT1+PIXOUT2) may beproportional to the time difference of “T_(tof)” and “T_(dly)” values,which are shown, for example, in FIG. 8 and discussed in more detaillater below. In case of the pixel 50 (or 67), for example, the “T_(tof)”parameter may be a pixel-specific TOF value of a light signal receivedby the PD 55 (or the PD 70) and the delay time parameter “T_(dly)” maybe the time from when the light signal 28 was initially transmitteduntil the VTX signal 99—in the TCC unit 64 (or the TCC unit 79)—startsto ramp. The delay time (T_(dly)) may be negative when the light pulse28 is transmitted after VTX 99 starts to ramp (which may typically occurwhen the electronic shutter 61 is “opened”). The above-mentionedproportionality relation may be represented by the following equation:

$\begin{matrix}{\frac{{Pixout}\; 1}{{{Pixout}\; 1} + {{Pixout}\; 2}} \propto \left( {T_{tof} - T_{dly}} \right)} & (1)\end{matrix}$

However, the present disclosure is not limited to the relationshippresent in equation (1). As discussed below, the ratio in equation (1)may be used to calculate depth or distance of a 3D object, and is lesssensitive to pixel-to-pixel variations when Pixout1+Pixout2 is notalways the same.

For ease of reference, the term “P1” may be used to refer to “Pixout1”and the term “P2” may be used to refer to “Pixout2” in the discussionbelow. It is seen from the relationship in equation (1) that thepixel-specific TOF value may be determined as a ratio of thepixel-specific output values P1 and P2. In certain embodiments, once thepixel-specific TOF value is so determined, the pixel-specific distance(“D”) or range (“R”) to an object (such as the 3D object 26 in FIG. 2)or a specific location on the object may be given by:

$\begin{matrix}{D = {T_{tof}*\frac{c}{2}}} & (2)\end{matrix}$

where the parameter “c” refers to the speed of light. Alternatively, insome other embodiments where the modulating signal—such as the VTXsignal 99 (or the TX signal 100) in FIG. 5, for example—is linear insidea shutter window, the range/distance may be computed as:

$\begin{matrix}{D = {\frac{c}{2}*\left\lbrack {\left( {\left( \frac{P_{1}}{P_{1} + P_{2}} \right)*T_{shutter}} \right) + T_{dly}} \right\rbrack}} & (3)\end{matrix}$

In equation (3), the parameter “T_(shutter)” is the shutter duration orshutter “ON” period. The parameter “T_(shutter)” is referred to as theparameter “T_(sh)” in the embodiments of FIGS. 8 and 10. Consequently, a3D image of the object—such as the object 26—may be generated by the TOFsystem 15 based on the pixel-specific range values determined as givenabove.

In view of the present disclosure's analog modulation-based manipulationor control of the PPD charge distribution inside a pixel itself, therange measurement and resolution are also controllable. The pixel-levelanalog amplitude modulation of the PPD charge may work with anelectronic shutter that may be a global shutter as, for example, in aCharge Coupled Device (CCD) image sensor. The global shutter may allowfor a better image capture of a fast-moving object (such as a vehicle),which may be helpful in a driver assistant system or an autonomousnavigation system. Furthermore, although the disclosure herein isprimarily provided in the context of a one-pulse TOF imaging system,like the system 15 in FIGS. 1-2, the principles of pixel-level internalanalog modulation approach discussed herein may be implemented, withsuitable modifications (if needed), in a continuous wave modulation TOFimaging system or a non-TOF system as well.

FIG. 6 is an exemplary timing diagram 109 that provides an overview ofthe modulated charge transfer mechanism in the TCC unit 84 of FIG. 5according to one embodiment of the present disclosure. The waveformsshown in FIG. 6 (and also in FIGS. 8 and 10) are simplified in natureand are for illustrative purpose only; the actual waveforms may differin timing as well as shape depending on the circuit implementation. Thesignals common between FIGS. 5 and 6 are identified using the samereference numerals for ease of comparison. These signals include theVPIX signal 104, the RST signal 98, the electronic SHUTTER signal 61,and the VTX modulating signal 99. Two additional waveforms 111-112 arealso shown in FIG. 6 to illustrate the status of the charge in PPD 89and that in the FD 102, respectively, when modulating signal 99 isapplied during charge transfer. In the embodiment of FIG. 6, VPIX 104may start as a low logic voltage (for example, logic 0 or 0 volts) toinitialize the pixel 50 (or 67) and switch to a high logic voltage (forexample, logic 1 or 3 volts (3V)) during operation of the pixel 50 (or67). RST 98 may start with a high logic voltage pulse (for example, apulse that goes from logic 0 to logic 1 and back to logic 0) during theinitialization of the pixel 50 (or 67) to set the charge in the PPD 89to its full well capacity and set the charge in the FD 102 to zeroCoulombs (0 C). The reset voltage level for FD 102 may be a logic 1level. During a range (TOF) measurement operation, the more electronsthe FD 102 receives from the PPD 89, the lower the voltage on the FD 102becomes. The Shutter signal 61 may start with a low logic voltage (forexample, logic 0 or 0V) during the initialization of the pixel 50 (or67), switch to a logic 1 level (for example, 3 volts) at a time thatcorresponds to the minimum measurement range during operation of thepixel 50 (or 67) to enable the PD 55 (or 70) to detect the photon(s) inthe returned light pulse 37 (represented as the incoming light signal 57in FIG. 3 and the incoming light signal 71 in FIG. 4), and then switchto a logic 0 level (for example, 0V) at a time that corresponds to themaximum measurement range. Thus, the duration of the logic 1 level ofthe shutter signal 61 may provide a pre-defined time interval/window toreceive the output from the PD 55 (or 70). The charge in the PPD 89starts out fully charged during initialization (when VPIX 104 is low,RST 98 is high, and VTX 99 is high to fill the charge in the PPD 89) anddecreases as VTX 99 is ramped from 0V to a higher voltage, preferably ina linear fashion. The PPD charge level under the control of the analogmodulating signal 99 is illustrated by waveform with reference numeral“111” in FIG. 6. The PPD charge decrease may be a function of how longVTX ramps, which results in a transfer of a certain amount of chargefrom the PPD 89 to the FD 102. Thus, as shown by the waveform withreference numeral “112” in FIG. 6, a charge in FD 102 starts out at alow charge (for example, 0 C) and increases as VTX 99 is ramped from 0Vto a higher voltage, which partially transfers a certain amount ofcharge from the PPD 89 to the FD 102. This charge transfer is a functionof how long VTX 99 ramps.

As noted before, the pixel-specific output (PIXOUT) 107 in FIG. 5 isderived from the PPD charge transferred to the floating diffusion node102. Thus, the Pixout signal 107 may be considered asamplitude-modulated over time by the analog modulating voltage VTX 99(or, equivalently, the TX voltage 100). In this manner, the TOFinformation is provided through Amplitude Modulation (AM) of thepixel-specific output 107 using the modulating signal VTX 99 (or,equivalently, the TX signal 100). In particular embodiments, themodulating function for generating the VTX signal 99 may be monotonic.In the exemplary embodiments of FIGS. 6, 8, and 10, the analogmodulating signals may be generated using a ramp function and, hence,they are shown as having ramp-type waveforms. However, in otherembodiments, different types of analog waveforms/functions may be usedas modulating signals.

FIG. 7 shows the block diagram of an exemplary logic unit 86 that may beused in the TCC unit 84 of FIG. 5 as per particular embodiments of thepresent disclosure. The logic unit 86 may include a latch 115 and atwo-input OR gate 116. While the shutter signal 61 is active or turned“on”, the latch 115 may receive the signal 87 from the relevantamplifier unit (for example, the sense amplifier's intermediate output62 or the gainstage's intermediate output 78) and may output a signalthat goes from logic 1 to logic 0 and remains at logic 0. In otherwords, the latch 115 converts the amplifier-provided signal 87—which isgenerated as a result of a photon detection event by the PD 55 or the PD70, as applicable—to a signal that goes from logic 1 to logic 0 andremains at logic 0, at least during the shutter ON period. In particularembodiments, the latch output may be triggered by the first edge of thesignal 87. The first edge may be positive-going or negative-goingdepending on the circuit design.

The two-input logic OR gate 116 may include a first input connected tothe output of the latch 115, a second input for receiving a signal(TXRMD) 117, and an output to provide the TXEN signal 96. In oneembodiment, the TXRMD signal 117 may be generated internally within therelevant pixel 50 (or 67). The OR gate 116 may logically OR the outputof the latch 115 with the TXRMD signal 117 to obtain the final TXENsignal 96. Such internally-generated signal may remain low while theelectronic shutter is “on”, but may be asserted “high” so that the TXENsignal 96 goes to a logic 1 to facilitate the transfer of the remainingcharge in the PPD 89 (at event 135 in FIG. 8, discussed below). In someembodiments, the TXRMD signal or a similar signal may beexternally-supplied.

FIG. 8 is a timing diagram 120 that shows exemplary timing of differentsignals in the system 15 of FIGS. 1-2 when the TCC unit 84 in theembodiment of FIG. 5 is used in a pixel, such as the pixel 50 or thepixel 67, as part of a pixel array, such as the pixel array 42 in FIG.2, for measuring TOF values according to certain embodiments of thepresent disclosure. Various signals—such as the transmitted pulse 28,the VPIX input 104, the TXEN input 96, and the like shown in theembodiments of FIGS. 2-5 are identified in FIG. 8 using the samereference numerals for the sake of consistency and ease of discussion.Prior to discussion FIG. 8, it is noted that, in the context of FIG. 8(and also in case of FIG. 10), the parameter “T_(dly)” refers to thetime delay between the rising edge of the projected pulse 28 and thetime instance when the VTX signal 99 starts to ramp, as indicated byreference numeral “122”; the parameter “T_(tof)” refers to thepixel-specific TOF value as measured by the delay between the risingedges of the projected pulse 28 and the received (returned) pulse 37, asindicated by reference numeral “123”; and the parameter “T_(sh)” refersto the time period between the “opening” and the “closing” of theelectronic shutter—as indicated by reference numeral “124” and given bythe assertion (for example, logic 1 or “on”) and de-assertion (orde-activation) (for example, logic 0 or “off”) of the shutter signal 61.Thus, the electronic shutter signal 61 is considered to be “active”during the period “T_(sh)”, which is also identified using the referencenumeral “125.” In some embodiments, the delay “T_(dly)” may bepre-determined and fixed regardless of operating conditions. In otherembodiments, the delay “T_(dly)” may be adjustable at run-time dependingon, for example, the external weather condition. It is noted here thatthe “high” or “low” signal levels relate to the design of the pixel 43(which is represented by the pixel 50 or 67). The signal polarities orbias levels shown in FIG. 8 may be different in other types of pixeldesigns based on, for example, the types of transistors or other circuitcomponents used.

As noted before, the waveforms shown in FIG. 8 (and also in FIG. 10) aresimplified in nature and are for illustrative purpose only; the actualwaveforms may differ in timing as well as shape depending on the circuitimplementation. As shown in FIG. 8, the returned pulse 37 may be atime-wise delayed version of the projected pulse 28. In particularembodiments, the projected pulse 28 may be of a very short duration suchas, for example, in the range of 5 to 10 nanoseconds (ns). The returnedpulse 37 may be sensed using a high gain PD in the pixel 43—such as thepixel 55 in the pixel 50 or the PD 70 in the pixel 67. The electronicshutter 61 may “control” the capture of the pixel-specific photon(s) inthe received light 37. The shutter signal 61 may have a gated delay—withreference to the projected pulse 28—to avoid the light scatters fromreaching the pixel array 42. The light scatters of the projected pulse28 may occur, for example, due to an inclement weather.

In addition to various external signals (for example, VPIX 104, RST 98,and the like) and internal signals (for example, TX 100, TXEN 96, and FDvoltage 102), the timing diagram 120 in FIG. 8 also identifies thefollowing events or time periods: (i) a PPD preset event 127 when RST,VTX, TXEN, and TX signals are high, while VPIX and SHUTTER signals arelow; (ii) a first FD reset event 128 from when TX is low until RST turnsfrom high to low; (iii) the delay time (T_(dly)) 122; (iv) the time offlight (T_(tof)) 123; (v) the electronic shutter “on” or “active” period(T_(sh)) 124; and (vi) a second FD reset event 130 for the duration ofwhen RST is a logic 1 for a second time. FIG. 8 also illustrates whenthe electronic shutter is “closed” or “off” initially (which isindicated by reference numeral “132”), when the electronic shutter is“open” or “on” (which is indicated by the reference numeral “125”), whenthe charge initially transferred to the FD node 102 is read out throughPIXOUT 107 (which is indicated by reference numeral “134”), when the FDvoltage is reset a second time at arrow 130, and when the remainingcharge in PPD 89 is transferred to FD 102 and again readout at event 135(for example, as output to PIXOUT 107). In one embodiment, the shutter“on” period (Tsh) may be less than or equal to the ramping time of VTX99.

Referring to FIG. 8, in case of the TCC unit 84 in FIG. 5, the PPD 89may be filled with charge to its full well capacity at an initializationstage (for example, the PPD Preset event 127). During the PPD presettime 127, the RST, VTX, TXEN, and TX signals may be high, whereas theVPIX and SHUTTER signals may be low, as shown. Then, the VTX signal 99(and, hence, the TX signal 100) may go low to shut off the secondtransistor 91 and the VPIX signal 104 may go high to commence the chargetransfer from the “fully-charged” PPD 89. In case of the electronicshutter 61 being a global shutter, in particular embodiments, all pixelsin the pixel array 42 may be selected together at a time and allselected PPDs may be reset together using the RST signal 98. Each pixelmay be read individually using the methodology similar to a frametransfer CCD or an inter-line transfer CCD. Each pixel-specific analogpixout signals (such as, for example, the pixout1 and pixout2 signals)may be sampled and converted to corresponding digital values—forexample, the earlier-mentioned “P1” and “P2” values—by an ADC unit (notshown).

In the embodiment shown in FIG. 8, all signals—except the TXEN signal96—start at logic 0 or “low” level as shown. Initially, as mentionedabove, the PPD 89 is preset when RST, VTX, TXEN, and TX go to a logic 1level, and VPIX stays low. Thereafter, the FD node 102 is reset whileRST is a logic 1, when VTX and TX go to a logic 0 and VPIX goes to high(or a logic 1). For ease of discussion, the same reference numeral “102”is used to refer to the FD node in FIG. 5 and associated voltagewaveform in the timing diagram of FIG. 8. After FD is reset to high (forexample, 0 C in charge domain), VTX is ramped while TXEN is a logic 1.The time of flight (Ttof) duration 123 is from when the laser pulse 28is transmitted until the returned pulse 37 is received, and is also thetime during which charge is transferred partially from the PPD 89 to theFD 102. The VTX input 99 (and, hence, the TX input 100) may be rampedwhile the shutter 61 is “on” or “open”. This may cause an amount ofcharge in the PPD 89 to be transferred to the FD 102, which may be afunction of how long VTX ramps. However, when the transmitted pulse 28reflects off of the object 26 and is received by a PD—such as the PD 55or the PD 70, depending on the pixel configuration, the generatedamplified output—such as the intermediate output signal 62 or theintermediate output signal 78, as applicable—may be processed by thelogic unit 86, which, in turn, may bring down the TXEN signal 96 to astatic logic 0. Thus, detection of the returned pulse 37 by a PD 55 (or70) in a temporally-correlated manner—that is, when the shutter is “on”or “active”—may be indicated by a logic 0 level for the TXEN signal 96.The logic low level of the TXEN input 96 turns off the first transistor90 and the second transistor 91, which stops the transfer of charge toFD 102 from the PPD 89. When SHUTTER input 61 goes to a logic 0 and SELinput 105 (not shown in FIG. 8) goes to a logic 1, the charge in FD 102is output as a voltage PIXOUT1 onto the PIXOUT line 107. Then, the FDnode 102 may be reset again (as indicated at reference numeral “130”)with a logic high RST pulse 98. Thereafter, when the TXEN signal 96 goesto a logic 1, the remaining charge in the PPD 89 is substantiallycompletely transferred to the FD node 102 and output as a voltagePIXOUT2 onto PIXOUT line 107. As mentioned earlier, the PIXOUT1 andPIXOUT2 signals may be converted into corresponding digital values P1and P2 by an appropriate ADC unit (not shown). In certain embodiments,these P1 and P2 values may be used in equation (2) or equation (3) aboveto determine a pixel-specific distance/range between a pixel 43 (asrepresented, for example, by the pixel 50 or 67) and the 3D object 26.

FIG. 9 shows circuit details of another exemplary TCC unit 140 as perparticular embodiments of the present disclosure. The TCC unit 140 maybe any of the TCC units 64 or 79. In some embodiments, the TCC unit 140may be used instead of the TCC unit 84 in FIG. 5. Although many signalsand circuit components are similar between the TCC units 84 (FIG. 5) and140 (FIG. 9), it does not imply that the TCC units in FIGS. 5 and 9 areidentical or that they operate in an identical manner. In view of theearlier discussion of FIG. 5, only a brief discussion of the TCC unit140 in FIG. 9 is provided to highlight its distinguishing aspects.

Like the TCC unit 84 in FIG. 5, the TCC unit 140 in FIG. 9 also includesa PPD 142, a logic unit 144, a first NMOS transistor 146, a second NMOStransistor 147, a third NMOS transistor 148, a fourth NMOS transistor149, a fifth NMOS transistor 150; generates the internal input TXEN 152;receives external inputs RST 154, VTX 156 (and, hence, the TX signal157), VPIX 159, and SEL 160; has an FD node 162; and outputs the PIXOUTsignal 165. However, unlike the TCC unit 84 in FIG. 5, the TCC unit 140in FIG. 9 also generates a second TXEN signal (TXENB) 167, which may bea complement of the TXEN signal 152 and may be supplied to the gateterminal of a sixth NMOS transistor 169. The sixth NMOS transistor 169may have its drain terminal connected to the source terminal of thetransistor 146 and its source terminal connected to a Ground (GND)potential 170. The TXENB signal 167 may be used to bring the GNDpotential to the gate terminal of the TX transistor 147. Without theTXENB signal 167, when the TXEN signal 152 goes low, the gate of the TXtransistor 147 may be floating and the charge transfer from the PPD 142may not be fully terminated. This situation may be ameliorated using theTXENB signal 167. Additionally, the TCC unit 140 also may include aStorage Diffusion (SD) capacitor 172 and a seventh NMOS transistor 174.The SD capacitor 172 may be connected at the junction of the drainterminal of the transistor 147 and the source terminal of transistor174, and may “form” an SD node 175 at the junction. The seventh NMOStransistor 174 may receive at its gate terminal a different, secondTransfer signal (TX2) 177 as an input. The drain of the transistor 174may connect to the FD node 162 as illustrated.

The signals RST, VTX, VPIX, TX2, and SEL may be supplied to the TCC unit140 from an external unit, such as, for example, the image processingunit 46 in FIG. 2. Furthermore, in certain embodiments, the SD capacitor172 may not be an extra capacitor, but may be merely the junctioncapacitor of the SD node 175. In the TCC unit 140, the charge transfertrigger portion may include the logic unit 144; the charge generationand transfer portion may include the PPD 142, the NMOS transistors146-148, 169, and 174, and the SD capacitor 172; and the chargecollection and output portion may include the NMOS transistors 148-150.It is noted here that separation of various circuit components intorespective portions is for illustrative and discussion purpose only. Incertain embodiments, such portions may include more or less or differentcircuit elements than those listed here. It is further noted that, likethe logic unit 86 in FIG. 7, the logic unit 144 also may receive thesignal 87 from the relevant amplifier unit—the sense amplifier 60 incase of the pixel 50 in FIG. 3 or the gainstage in case of the pixel 67in FIG. 4. The signal 87 may represent either of the intermediateoutputs 62 and 78, as applicable. In certain embodiments, the logic unit144 may be a modified version of the logic unit 86 in FIG. 7 to provideboth the TXEN 152 and TXENB 167 outputs.

It is observed that the configuration of the TCC unit 140 in FIG. 9 issubstantially similar to that of the TCC unit 84 in FIG. 5. Therefore,for the sake of brevity, the circuit portions and signals common betweenthe embodiments in FIGS. 5 and 9—such as the transistors 146-150 andassociated inputs like RST, SEL, VPIX, and so on—are not discussed here.It is observed that the TCC unit 140 in FIG. 9 may allow for aCorrelated Double Sampling (CDS) based charge transfer. The CDS is anoise reduction technique for measuring an electrical value, such as apixel/sensor output voltage (pixout), in a manner that allows removal ofan undesired offset. In CDS, the output(s) of a pixel, such as thePixout 165 in FIG. 9, may be measured twice—once in a known condition,and once in an unknown condition. The value measured from the knowncondition may be then subtracted from the value measured from theunknown condition to generate a value with a known relation to thephysical quantity being measured—here, the PPD charge representing thepixel-specific portion of the received light. Using CDS, noise may bereduced by removing the reference voltage of the pixel (such as, forexample, the pixel's voltage after it is reset) from the signal voltageof the pixel at the end of each charge transfer. Thus, in CDS, beforethe charge of a pixel is transferred as an output, the reset/referencevalue is sampled, which is then “deducted” from the value after thecharge of the pixel is transferred.

In the embodiment of FIG. 9, the SD capacitor 172 (or the associated SDnode 175) stores the PPD charge prior to its transfer to the FD node162, thereby allowing the establishment (and sampling) of appropriatereset values at the FD node 162 prior to any charge is transferred tothe FD node 162. As a result, each pixel-specific output (Pixout1 andPixout2) may be processed by a CDS unit (not shown) in the imageprocessing unit 46 (FIG. 2) to obtain a pair of pixel-specific CDSoutputs. Subsequently, the pixel-specific CDS outputs may be convertedto digital values—here, the P1 and P2 values mentioned earlier—by an ADCunit (not shown) in the image processing unit 46 (FIG. 2). Thetransistors 169 and 174, and the signals TXENB 167 and TX2 177 in FIG. 9provide ancillary circuit components needed to facilitate CDS-basedcharge transfer. In one embodiment, the P1 and P2 values may begenerated in parallel using, for example, an identical pair of ADCcircuits. Thus, the differences between the reset levels andcorresponding PPD charge levels of pixout1 and pixout2 signals may beconverted to digital numbers by an ADC unit (not shown) and output asthe pixel-specific signal values—P1 and P2—to enable the computation ofthe pixel-specific TOF value of the returned pulse 37 for the pixel 43(as represented, for example, by pixels 50 or 67) based on the equation(1) given before. As noted earlier, such computation may be performed bythe image processing unit 46 itself or by the processor 19 in the system15. Consequently, a pixel-specific distance to the 3D object 26 (FIG. 2)also may be determined using, for example, equation (2) or equation (3).The pixel-by-pixel charge collection operation may be performed for allpixels in the pixel array 42. Based on all the pixel-specific distanceor range values for the pixels 43 in the pixel array 42, a 3D image ofthe object 26 may be generated, for example, by the processor 19, anddisplayed on an appropriate display or user interface associated withthe system 15. Furthermore, a 2D image of the 3D object 26 may begenerated—for example, when no range values are calculated or when a 2Dimage is desired despite the availability of range values by simplyadding the P1 and P2 values. In particular embodiments, such a 2D imagesimply may be a grayscale image, for example, when an IR laser is used.

It is observed here that the pixel configurations shown in FIGS. 3-4 aswell as the TCC configurations shown in FIGS. 5 and 9 are exemplaryonly. As mentioned before, pixels with multiple, high-gain PDs also maybe used to implement the teachings of the present disclosure. Similarly,a non-PPD based TCC unit also may be selected for a pixel (such as thepixel 43 in FIG. 2) as per teachings of the present disclosure.Furthermore, in some embodiments, the TCC units may have a single output(such as the PIXOUT lines 107, 165 in the embodiments of FIGS. 5 and 9,respectively) or, in other embodiments, the TCC units may have dualoutputs where Pixout1 and Pixout2 signals may be output throughdifferent output lines (not shown). It is noted here that the pixelconfigurations 50, 67 discussed herein may be CMOS configurations. Inother words, each pixel-specific PD unit, amplifier unit, and TCC unitmay be a CMOS portion. As a result, DTOF measurements and rangedetection operations may be performed at a substantially lower voltageand higher PDE than the existing SPAD or APD based systems.

FIG. 10 is a timing diagram 180 that shows exemplary timing of differentsignals in the system 15 of FIGS. 1-2 when the TCC unit 140 in theembodiment of FIG. 9 is used in a pixel, such as the pixel 50 or thepixel 67, as part of a pixel array, such as the pixel array 42 in FIG.2, for measuring TOF values according to certain embodiments of thepresent disclosure. The timing diagram 180 in FIG. 10 is similar to thetiming diagram 120 in FIG. 8—especially with reference to the waveformsof VTX, Shutter, VPIX, and TX signals, and identification of varioustiming intervals or events such as, for example, the PPD preset event,the shutter “on” period, the time delay period (T_(dly)), and so on.Because of the earlier extensive discussion of the timing diagram 120 inFIG. 8, only a brief discussion of the distinguishing features in thetiming diagram 180 in FIG. 10 is provided for the sake of brevity.

In FIG. 10, for the sake of consistency and ease of discussion, variousexternally-supplied signals—such as the VPIX signal 159, the RST signal154, the electronic shutter signal 61, the analog modulating signal VTX156, and the TX2 signal 177—and the internally-generated TXEN signal 152are identified using the same reference numerals as those used for thesesignals in FIG. 9. Similarly, for ease of discussion, the same referencenumeral “162” is used to refer to the FD node in FIG. 9 and associatedvoltage waveform in the timing diagram of FIG. 10. A Transfer Mode(TXRMD) signal 182 is shown in FIG. 10 (and a similar signal is alsomentioned in FIG. 7), but not shown in FIG. 9 or in the earlier timingdiagram of FIG. 8. In particular embodiments, the TXRMD signal 182 maybe internally generated by the logic unit 144 or externally-supplied tothe logic unit 144, for example, by the image processing unit 46 (FIG.2). Like the logic unit 86 in FIG. 7, in one embodiment, the logic unit144 may include logic circuits (not shown) to generate an output andthen logically OR the output with an internally-generated signal—suchas, for example, the TXRMD signal 182—to obtain the final TXEN signal152. As shown in FIG. 10, in one embodiment, such internally-generatedTXRMD signal 182 may remain low while the electronic shutter is “on”,but may be asserted “high” thereafter so that the TXEN signal 152 goesto a logic 1 to facilitate the transfer of the remaining charge in thePPD 142 (at event 183 in FIG. 10).

It is noted that the PPD preset event 184, the delay time (T_(dly)) 185,the TOF period (T_(tof)) 186, the shutter “off” interval 187, and theshutter “on” or “active” period (T_(sh)) 188 or 189, and the FD resetevent 190 in FIG. 10 are similar to corresponding events or time periodsshown in FIG. 8. Therefore, additional discussion of these parameters isnot provided for the sake of brevity. Initially, the FD reset event 190results in the FD signal 162 going “high”, as shown. The SD node 175 isreset to “high” after the PPD 142 is preset to “low”. More specifically,during the PPD preset event 184, the TX signal 157 may be “high”, theTX2 signal 177 may be “high”, the RST signal 154 may be “high”, and theVPIX signal 159 may be “low” to fill electrons to PPD 142 and preset itto zero volt. Thereafter, the TX signal 157 may go “low” but the TX2signal 177 and the RST signal 154 may briefly remain “high”, which,along with a “high” VPIX signal 159, may reset the SD node 175 to “high”and remove electrons from the SD capacitor 172. In the meantime, the FDnode 162 is reset as well (following the FD reset event 190). Thevoltage at the SD node 175 or the SD reset event are not shown in FIG.10.

In contrast to the embodiment in FIGS. 6 and 8, the PPD charge isamplitude modulated and initially transferred to the SD node 175(through the SD capacitor 172) in the embodiment of FIGS. 9-10 when theelectronic shutter 61 is “active” and the VTX signal 156 is ramped up—asnoted on the TX waveform 157. Upon detection of photons by the high-gainPD—such as the PD 55 or the PD 70, as applicable—during the shutter “on”period 189, the TXEN signal 152 goes “low” and the initial chargetransfer from the PPD 142 to the SD node 175 stops. The transferredcharge stored at the SD node 175 may be read out on the Pixout line 165(as a Pixout1 output) during the first readout period 191. In the firstreadout period 191, the RST signal 154 may be briefly asserted “high”after the electronic shutter 61 is de-activated or turned “off” to resetthe FD node 162. Thereafter, the TX2 signal 177 may be pulsed “high” totransfer the charge from the SD node 175 to the FD node 162 while TX2 is“high”. The FD voltage waveform 162 illustrates this charge transferoperation. The transferred charge then may be readout (as Pixout1voltage) during the first readout period 191 via the Pixout line 165using the SEL signal 160 (not shown in FIG. 10).

During the first readout interval 191, after the initial charge istransferred from the SD node to the FD node and the TX2 signal 177returns to the logic “low” level, the TXRMD signal 182 may be asserted(pulsed) “high” to generate a “high” pulse on the TXEN input 152, which,in turn, may generate a “high” pulse on the TX input 157 to allowtransfer of the remaining charge in the PPD 142 to the SD node 175(through the SD capacitor 172)—as indicated by the reference numeral“183” in FIG. 10. Thereafter, the FD node 162 may be reset again whenthe RST signal 154 is briefly asserted “high” again. The second RST highpulse may define a second readout period 192, in which the TX2 signal177 may be pulsed “high” again “to transfer the PPD's remainder charge(at event 183) from the SD node 175 to the FD node 162 while TX2 is“high”. The FD voltage waveform 162 illustrates this second chargetransfer operation. The transferred remaining charge then may be readout(as Pixout2 voltage) during the second readout period 192 via the Pixoutline 165 using the SEL signal 160 (not shown in FIG. 10). As mentionedearlier, the PIXOUT1 and PIXOUT2 signals may be converted intocorresponding digital values P1 and P2 by an appropriate ADC unit (notshown). In certain embodiments, these P1 and P2 values may be used inequation (2) or equation (3) above to determine a pixel-specificdistance/range between the pixel 43 and the 3D object 26. The SD-basedcharge transfer illustrated in FIG. 10 allows for a generation of a pairof pixel-specific CDS outputs, as discussed earlier with reference todiscussion of FIG. 9. The CDS-based signal processing provides foradditional noise reduction, as also mentioned before.

In summary, the pixel designs as per teachings of the present disclosureuse one or more high-gain PDs in combination with a PPD (or similaranalog charge storage device), which performs as a time-to-chargeconverter whose AM-based charge transfer operation is controlled byoutputs from the one or more high-gain PDs in the pixel to determineTOF. In the present disclosure, the PPD charge transfer is stopped torecord TOF only when an output from a high-gain PD is triggered within avery short, pre-defined time interval—such as, for example, when anelectronic shutter is “on.” As a result, an all-weather autonomousnavigation system as per teachings of the present disclosure may provideimproved vision for drivers under difficult driving conditions such as,for example, low light, fog, bad weather, and so on.

FIG. 11 depicts an exemplary flowchart 195 showing how a TOF value maybe determined in the system 15 of FIGS. 1-2 according to one embodimentof the present disclosure. Various steps illustrated in FIG. 11 may beperformed by a single module or a combination of modules or systemcomponents in the system 15. In the discussion herein, by way of anexample only, specific tasks are described as being performed byspecific modules or system components. Other modules or systemcomponents may be suitably configured to perform such tasks as well. Asnoted at block 197, initially, the system 15 (more specifically, theprojector module 22) may project a laser pulse, such as the pulse 28 inFIG. 2, onto a 3D object, like the object 26 in FIG. 2. At block 198,the processor 19 (or the image processing unit 46 in certainembodiments) may apply an analog modulating signal, such as the VTXsignal 99 in FIG. 6, to a device in a pixel, such as the PPD 89 in thepixel 50 or 67 (as per design choice). As mentioned earlier, the pixel50 or 67 may be any of the pixels 43 in the pixel array 42 in FIG. 2.Furthermore, as noted at block 198, the device—such as the PPD 89—may beoperable to store an analog charge. At block 199, the image processingunit 46 may initiate transfer of a portion of the analog charge from thedevice (like the PPD 89) based on modulation received from the analogmodulating signal, such as the VTX signal 99. To initiate such chargetransfer, the image processing unit 46 may provide various externalsignals—such as the shutter signal 61, the VPIX signal 104, and the RSTsignal 98—to the relevant pixel 50 or 67 at the logic levels illustratedin the exemplary timing diagram of FIG. 6.

At block 200, a returned pulse, such as the returned pulse 37, may bedetected using the pixel 50 (or 67). As mentioned earlier, the returnedpulse 37 is the projected laser pulse 28 reflected from the 3D object26. As noted at block 200, the pixel 50 (or 67) may include a PDunit—such as the PD unit 52 (or the PD unit 68)—having at least one PD,like the PD 55 (or the PD 70), that converts luminance received in thereturned pulse 37 into an electrical signal and that has a conversiongain that satisfies a threshold. In particular embodiments, thethreshold is at least 400 μV per photon, as mentioned before. As notedat block 201, this electrical signal may be processed using an amplifierunit—such as the sense amplifier 60 (or the gainstage in the output unit69)—in the pixel 50 (or 67) to responsively generate an intermediateoutput. In the embodiment of FIG. 3, such intermediate output isrepresented by the line 62, whereas it is represented by the line 78 inthe embodiment of FIG. 4. As noted with reference to discussion of FIGS.5 and 9, the relevant logic unit 86 (FIG. 5) or 144 (FIG. 9) (as perdesign choice) may process the intermediate output 87 (which may be theoutput at line 62 or 78, as applicable) and may place the TXEN signal 96(FIG. 5) or 152 (FIG. 9) in the logic 0 (low) state. The logic 0 levelof the TXEN signal 96 or 152 turns off the first transistor 90 and thesecond transistor 91 in the TCC unit 84 in FIG. 5 (or the correspondingtransistors 146-147 in the TCC unit 140 in FIG. 9), which stops thetransfer of charge to corresponding FD node 102 (or 162) from the PPD 89(or 142). Thus, at block 202, the circuit in the relevant TCC unit 84(or 140) may terminate the earlier-initiated transfer of the portion ofthe analog charge (at block 199) in response to generation of theintermediate output 87 within a pre-defined time interval—such as, forexample, within the shutter “on” period 125 in FIG. 8 (or thecorresponding period 189 in FIG. 10).

As discussed earlier with reference to FIGS. 5 and 9, the portion of thecharge transferred to the respective FD node 102 (FIG. 5) or 162 (FIG.9) (until the transfer is terminated at block 202) may be read out as aPixout1 signal and converted into an appropriate digital value “P1”. Thedigital value “P1” may be used—along with a subsequently-generateddigital value “P2” (for Pixout2 signal)—to obtain the TOF informationfrom the ratio P1/(P1+P2), as outlined before. Thus, as noted at block203, either the image processing unit 46 or the processor 19 in thesystem 15 may determine the TOF value of the returned pulse 37 based onthe portion of the analog charge transferred upon termination (at block202).

FIG. 12 depicts an overall layout of the system 15 in FIGS. 1-2according to one embodiment of the present disclosure. Hence, for easeof reference and discussion, the same reference numerals are used inFIGS. 1-2 and 12 for the common system components/units.

As discussed earlier, the imaging module 17 may include the desiredhardware shown in the exemplary embodiments of FIGS. 3-5, 7, and 9, asapplicable, to accomplish 2D/3D imaging and TOF measurements as per theinventive aspects of the present disclosure. The processor 19 may beconfigured to interface with a number of external devices. In oneembodiment, the imaging module 17 may function as an input device thatprovides data inputs—in the form of processed pixel outputs such as, forexample, the P1 and P2 values—to the processor 19 for furtherprocessing. The processor 19 may also receive inputs from other inputdevices (not shown) that may be part of the system 15. Some examples ofsuch input devices include a computer keyboard, a touchpad, atouch-screen, a joystick, a physical or virtual “clickable button,”and/or a computer mouse/pointing device. In FIG. 12, the processor 19 isshown coupled to the system memory 20, a peripheral storage unit 206,one or more output devices 207, and a network interface unit 208. InFIG. 12, a display unit is shown as an output device 207. In someembodiments, the system 15 may include more than one instance of thedevices shown. Some examples of the system 15 include a computer system(desktop or laptop), a tablet computer, a mobile device, a cellularphone, a video gaming unit or console, a machine-to-machine (M2M)communication unit, a robot, an automobile, a virtual reality equipment,a stateless “thin” client system, a car's dash-cam or rearview camerasystem, an autonomous navigation system, or any other type of computingor data processing device. In various embodiments, all of the componentsshown in FIG. 12 may be housed within a single housing. Thus, the system15 may be configured as a standalone system or in any other suitableform factor. In some embodiments, the system 15 may be configured as aclient system rather than a server system. In particular embodiments,the system 15 may include more than one processor (e.g., in adistributed processing configuration). When the system 15 is amultiprocessor system, there may be more than one instance of theprocessor 19 or there may be multiple processors coupled to theprocessor 19 via their respective interfaces (not shown). The processor19 may be a System on Chip (SoC) and/or may include more than oneCentral Processing Unit (CPU).

As mentioned earlier, the system memory 20 may be anysemiconductor-based storage system such as, for example, DRAM, SRAM,PRAM, RRAM, CBRAM, MRAM, STT-MRAM, and the like. In some embodiments,the memory unit 20 may include at least one 3DS memory module inconjunction with one or more non-3DS memory modules. The non-3DS memorymay include Double Data Rate or Double Data Rate 2, 3, or 4 SynchronousDynamic Random Access Memory (DDR/DDR2/DDR3/DDR4 SDRAM), or Rambus®DRAM, flash memory, various types of Read Only Memory (ROM), etc. Also,in some embodiments, the system memory 20 may include multiple differenttypes of semiconductor memories, as opposed to a single type of memory.In other embodiments, the system memory 20 may be a non-transitory datastorage medium.

The peripheral storage unit 206, in various embodiments, may includesupport for magnetic, optical, magneto-optical, or solid-state storagemedia such as hard drives, optical disks (such as Compact Disks (CDs) orDigital Versatile Disks (DVDs)), non-volatile Random Access Memory (RAM)devices, flash memories, and the like. In some embodiments, theperipheral storage unit 206 may include more complex storagedevices/systems such as disk arrays (which may be in a suitable RAID(Redundant Array of Independent Disks) configuration) or Storage AreaNetworks (SANs), and the peripheral storage unit 206 may be coupled tothe processor 19 via a standard peripheral interface such as a SmallComputer System Interface (SCSI) interface, a Fibre Channel interface, aFirewire® (IEEE 1394) interface, a Peripheral Component InterfaceExpress (PCI Express™) standard based interface, a Universal Serial Bus(USB) protocol based interface, or another suitable interface. Varioussuch storage devices may be non-transitory data storage media.

The display unit 207 may be an example of an output device. Otherexamples of an output device include a graphics/display device, acomputer screen, an alarm system, a CAD/CAM (Computer AidedDesign/Computer Aided Machining) system, a video game station, asmartphone display screen, a dashboard-mounted display screen in anautomobile, or any other type of data output device. In someembodiments, the input device(s), such as the imaging module 17, and theoutput device(s), such as the display unit 207, may be coupled to theprocessor 19 via an I/O or peripheral interface(s).

In one embodiment, the network interface 208 may communicate with theprocessor 19 to enable the system 15 to couple to a network (not shown).In another embodiment, the network interface 208 may be absentaltogether. The network interface 208 may include any suitable devices,media and/or protocol content for connecting the system 15 to anetwork—whether wired or wireless. In various embodiments, the networkmay include Local Area Networks (LANs), Wide Area Networks (WANs), wiredor wireless Ethernet, the Internet, telecommunication networks,satellite links, or other suitable types of network.

The system 15 may include an on-board power supply unit 210 to provideelectrical power to various system components illustrated in FIG. 12.The power supply unit 210 may receive batteries or may be connectable toan AC electrical power outlet or an automobile-based power outlet. Inone embodiment, the power supply unit 210 may convert solar energy orother renewable energy into electrical power.

In one embodiment, the imaging module 17 may be integrated with ahigh-speed interface such as, for example, a Universal Serial Bus 2.0 or3.0 (USB 2.0 or 3.0) interface or above, that plugs into any PersonalComputer (PC) or laptop. A non-transitory, computer-readable datastorage medium, such as, for example, the system memory 20 or aperipheral data storage unit such as a CD/DVD may store program code orsoftware. The processor 19 and/or the image processing unit 46 (FIG. 2)in the imaging module 17 may be configured to execute the program code,whereby the device 15 may be operative to perform the 2D imaging (forexample, grayscale image of a 3D object), TOF and range measurements,and generation of a 3D image of an object using the pixel-specificdistance/range values, as discussed hereinbefore—such as, for example,the operations discussed earlier with reference to FIGS. 1-11. Forexample, in certain embodiments, upon execution of the program code, theprocessor 19 and/or the image processing unit 46 may suitably configure(or activate) relevant circuit components to apply appropriate inputsignals, like the Shutter, RST, VTX, SEL signals, and so on, to thepixels 43 in the pixel array 42 to enable capture of the light from areturned laser pulse and to subsequently process the pixel outputs forpixel-specific P1 and P2 values needed for TOF and range measurements.The program code or software may be proprietary software or open sourcesoftware which, upon execution by the appropriate processing entity—suchas the processor 19 and/or the image processing unit 46—may enable theprocessing entity to process various pixel-specific ADC outputs (P1 andP2 values), determine range values, render the results in a variety offormats including, for example, displaying a 3D image of the distantobject based on TOF-based range measurements. In certain embodiments,the image processing unit 46 in the imaging module 17 may perform someof the processing of pixel outputs before the pixel output data are sentto the processor 19 for further processing and display. In otherembodiments, the processor 19 also may perform some or all of thefunctionality of the image processing unit 46, in which case, the imageprocessing unit 46 may not be a part of the imaging module 17.

In the preceding description, for purposes of explanation and notlimitation, specific details are set forth (such as particulararchitectures, waveforms, interfaces, techniques, etc.) in order toprovide a thorough understanding of the disclosed technology. However,it will be apparent to those skilled in the art that the disclosedtechnology may be practiced in other embodiments that depart from thesespecific details. That is, those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the disclosed technology. In someinstances, detailed descriptions of well-known devices, circuits, andmethods are omitted so as not to obscure the description of thedisclosed technology with unnecessary detail. All statements hereinreciting principles, aspects, and embodiments of the disclosedtechnology, as well as specific examples thereof, are intended toencompass both structural and functional equivalents thereof.Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, such as, for example, any elements developed that perform thesame function, regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat block diagrams herein (e.g., in FIGS. 1-2 and 12) can representconceptual views of illustrative circuitry or other functional unitsembodying the principles of the technology. Similarly, it will beappreciated that the flowchart in FIG. 11 represents various processeswhich may be substantially performed by a processor (e.g., the processor19 and/or the image processing unit 46 in FIG. 2) in conjunction withvarious system components such as, for example, the projector module 22,the 2D pixel array 42, and the like. Such a processor may include, byway of example, a general purpose processor, a special purposeprocessor, a conventional processor, a digital signal processor (DSP), aplurality of microprocessors, one or more microprocessors in associationwith a DSP core, a controller, a microcontroller, Application SpecificIntegrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs)circuits, any other type of integrated circuit (IC), and/or a statemachine. Some or all of the processing functionalities described abovein the context of FIGS. 1-12 also may be provided by such a processor,in the hardware and/or software.

When certain inventive aspects require software-based processing, suchsoftware or program code may reside in a computer-readable data storagemedium. As noted earlier, such data storage medium may be part of theperipheral storage 206, or may be part of the system memory 20 or anyinternal memory (not shown) of the image sensor unit 24, or theprocessor's 19 internal memory (not shown). In one embodiment, theprocessor 19 and/or the image processing unit 46 may executeinstructions stored on such a medium to carry out the software-basedprocessing. The computer-readable data storage medium may be anon-transitory data storage medium containing a computer program,software, firmware, or microcode for execution by a general purposecomputer or a processor mentioned above. Examples of computer-readablestorage media include a ROM, a RAM, a digital register, a cache memory,semiconductor memory devices, magnetic media such as internal harddisks, magnetic tapes and removable disks, magneto-optical media, andoptical media such as CD-ROM disks and DVDs.

Alternative embodiments of the imaging module 17 or the system 15comprising such an imaging module according to inventive aspects of thepresent disclosure may include additional components responsible forproviding additional functionality, including any of the functionalityidentified above and/or any functionality necessary to support thesolution as per the teachings of the present disclosure. Althoughfeatures and elements are described above in particular combinations,each feature or element can be used alone without the other features andelements or in various combinations with or without other features. Asmentioned before, various 2D and 3D imaging functions discussed hereinmay be provided through the use of hardware (such as circuit hardware)and/or hardware capable of executing software/firmware in the form ofcoded instructions or microcode stored on a computer-readable datastorage medium (mentioned above). Thus, such functions and illustratedfunctional blocks are to be understood as being eitherhardware-implemented and/or computer-implemented, and thusmachine-implemented.

The foregoing describes a system and method in which a DTOF technique iscombined with analog amplitude modulation (AM) within each pixel in apixel array. No SPADs or APDs are used in the pixels. Instead, eachpixel has a PD with a conversion gain of over 400 μV/e− and PDE of morethan 45%, operating in conjunction with a PPD (or a similar analogstorage device). The TOF information is added to the received lightsignal by the analog domain-based single-ended to differential converterinside the pixel itself. The output of the PD in a pixel is used tocontrol the operation of the PPD. The charge transfer from the PPD isstopped—and, hence, TOF value and range of an object are recorded—whenthe output from the PD in the pixel is triggered within a pre-definedtime interval. Such pixels provide for an improved autonomous navigationsystem—with an AM-based DTOF sensor—for drivers under difficult drivingconditions such as, for example, low light, fog, bad weather, and so on.

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a wide range of applications. Accordingly, the scope of patentedsubject matter should not be limited to any of the specific exemplaryteachings discussed above, but is instead defined by the followingclaims.

What is claimed is:
 1. A pixel in an image sensor, said pixelcomprising: a Photo Diode (PD) unit having at least one PD that convertsreceived luminance into an electrical signal, wherein the at least onePD has a conversion gain that satisfies a threshold; an amplifier unitconnected in series with the PD unit to amplify the electrical signaland to responsively generate an intermediate output; and aTime-to-Charge Converter (TCC) unit coupled to the amplifier unit andreceiving the intermediate output therefrom, wherein the TCC unitincludes: a device that stores an analog charge, and a control circuitcoupled to the device, wherein the control circuit performs operationscomprising: initiating transfer of a first portion of the analog chargefrom the device, terminating the transfer in response to receipt of theintermediate output within a pre-defined time interval, and generating afirst pixel-specific output for the pixel based on the first portion ofthe analog charge transferred.
 2. The pixel of claim 1, wherein each ofthe PD unit, the amplifier unit, and the TCC unit comprises aComplementary Metal Oxide Semiconductor (CMOS) portion.
 3. The pixel ofclaim 1, wherein the PD unit includes: a first PD that receives theluminance and generates the electrical signal in response thereto,wherein the first PD has the conversion gain that satisfies thethreshold; and a second PD connected in parallel to the first PD,wherein the second PD is unexposed to the luminance and generates areference signal based on a level of darkness detected thereby.
 4. Thepixel of claim 3, wherein the amplifier unit includes: a sense amplifierconnected in series with the first and the second PDs to amplify theelectrical signal upon sensing the electrical signal vis-à-vis thereference signal, wherein the sense amplifier generates the intermediateoutput upon amplifying the electrical signal in response to a controlsignal received thereby.
 5. The pixel of claim 4, wherein the senseamplifier is a current sense amplifier.
 6. The pixel of claim 1, whereinthe device is one of the following: a Pinned Photo Diode (PPD); aphotogate; and a capacitor.
 7. The pixel of claim 1, wherein the controlcircuit includes an output terminal, and wherein the control circuitfurther performs the operations comprising: receiving an analogmodulating signal; further receiving an external input; transferring thefirst portion of the analog charge as the first pixel-specific outputthrough the output terminal in response to the external input and basedon modulation provided by the analog modulating signal; and transferringa second portion of the analog charge as a second pixel-specific outputthrough the output terminal in response to the external input, whereinthe second portion is substantially equal to a remainder of the analogcharge after the first portion is transferred.
 8. The pixel of claim 7,wherein the control circuit includes a first node and a second node, andwherein the control circuit further performs the operations comprising:transferring the first portion of the analog charge from the device tothe first node, from the first node to the second node, and from thesecond node to the output terminal as the first pixel-specific output;and transferring the second portion of the analog charge from the deviceto the first node, from the first node to the second node, and from thesecond node to the output terminal as the second pixel-specific output.9. The pixel of claim 1, wherein the threshold is at least 400 μV perphotoelectron.
 10. A method comprising: projecting a laser pulse onto athree-dimensional (3D) object; applying an analog modulating signal to adevice in a pixel, wherein the device stores an analog charge;initiating transfer of a first portion of the analog charge from thedevice based on modulation received from the analog modulating signal;detecting a returned pulse using the pixel, wherein the returned pulseis the projected laser pulse reflected from the 3D object, and whereinthe pixel includes a Photo Diode (PD) unit having at least one PD thatconverts luminance received in the returned pulse into an electricalsignal and that has a conversion gain that satisfies a threshold;processing the electrical signal using an amplifier unit in the pixel toresponsively generate an intermediate output; terminating the transferof the first portion of the analog charge in response to generation ofthe intermediate output within a pre-defined time interval; anddetermining a Time of Flight (TOF) value of the returned pulse based onthe first portion of the analog charge transferred upon termination. 11.The method of claim 10, further comprising: generating a firstpixel-specific output of the pixel from the first portion of the analogcharge transferred from the device; transferring a second portion of theanalog charge from the device, wherein the second portion issubstantially equal to a remainder of the analog charge after the firstportion is transferred; generating a second pixel-specific output of thepixel from the second portion of the analog charge transferred from thedevice; sampling the first and the second pixel-specific outputs usingan Analog-to-Digital Converter (ADC) unit; and based on the sampling,generating a first signal value corresponding to the firstpixel-specific output and a second signal value corresponding to thesecond pixel-specific output using the ADC unit.
 12. The method of claim11, further comprising: determining the TOF value of the returned pulseusing a ratio of the first signal value to a total of the first and thesecond signal values.
 13. The method of claim 12, further comprising:determining a distance to the 3D object based on the TOF value.
 14. Themethod of claim 10, further comprising: further applying a shuttersignal to the amplifier unit, wherein the shutter signal is applied apre-determined time period after projecting the laser pulse; detectingthe returned pulse using the pixel while the shutter signal as well asthe analog modulating signal are active; providing a termination signalupon generation of the intermediate output while the shutter signal isactive; and terminating the transfer of the first portion of the analogcharge in response to the termination signal.
 15. The method of claim10, wherein detecting the returned pulse includes: receiving theluminance at a first PD in the PD unit, wherein the first PD has theconversion gain that satisfies the threshold; generating the electricalsignal using the first PD; and further generating a reference signalusing a second PD in the PD unit, wherein the second PD is connected inparallel to the first PD, is unexposed to the luminance, and generatesthe reference signal based on a level of darkness detected thereby. 16.The method of claim 15, wherein the amplifier unit is a sense amplifierconnected in series with the first and the second PDs, and whereinprocessing the electrical signal includes: providing a shutter signal tothe sense amplifier; sensing the electrical signal vis-à-vis thereference signal using the sense amplifier while the shutter signal isactive; and generating the intermediate output by amplifying theelectrical signal using the sense amplifier while the shutter signal isactive.
 17. The method of claim 10, wherein projecting the laser pulseincludes: projecting the laser pulse using a light source that is one ofthe following: a laser light source; a light source that produces lightin a visible spectrum; a light source that produces light in anon-visible spectrum; a monochromatic illumination source; an Infrared(IR) laser; an X-Y addressable light source; a point source withtwo-dimensional (2D) scanning capability; a sheet source withone-dimensional (1D) scanning capability; and a diffused laser.
 18. Themethod of claim 10, wherein the threshold is at least 400 μV per photon.19. A system comprising: a light source that projects a laser pulse ontoa three-dimensional (3D) object; a plurality of pixels, wherein eachpixel includes: a pixel-specific Photo Diode (PD) unit having at leastone PD that converts luminance received in a returned pulse into anelectrical signal, wherein the at least one PD has a conversion gainthat satisfies a threshold, and wherein the returned pulse results fromreflection of the projected laser pulse by the 3D object, apixel-specific amplifier unit connected in series with thepixel-specific PD unit to amplify the electrical signal and toresponsively generate an intermediate output, and a pixel-specificTime-to-Charge Converter (TCC) unit coupled to the pixel-specificamplifier unit and receiving the intermediate output therefrom, whereinthe pixel-specific TCC unit includes: a device that stores an analogcharge, and a control circuit coupled to the device, wherein the controlcircuit performs operations comprising: initiating transfer of apixel-specific first portion of the analog charge from the device,terminating the transfer of the pixel-specific first portion uponreceipt of the intermediate output within a pre-defined time interval,generating a first pixel-specific output for the pixel based on thepixel-specific first portion of the analog charge transferred,transferring a pixel-specific second portion of the analog charge fromthe device, wherein the pixel-specific second portion is substantiallyequal to a remainder of the analog charge after the pixel-specific firstportion is transferred, and generating a second pixel-specific outputfor the pixel based on the pixel-specific second portion of the analogcharge transferred; a memory for storing program instructions; and aprocessor coupled to the memory and to the plurality of pixels, whereinthe processor executes the program instructions, whereby the processorperforms the following operations for each pixel in the plurality ofpixels: facilitating transfers of the pixel-specific first and secondportions of the analog charge, respectively, receiving the first and thesecond pixel-specific outputs, generating a pixel-specific pair ofsignal values based on the first and the second pixel-specific outputs,respectively, wherein the pixel-specific pair of signal values includesa pixel-specific first signal value and a pixel-specific second signalvalue, determining a corresponding pixel-specific Time of Flight (TOF)value of the returned pulse using the pixel-specific first signal valueand the pixel-specific second signal value, and determining apixel-specific distance to the 3D object based on the pixel-specific TOFvalue.
 20. The system of claim 19, wherein the processor provides ananalog modulating signal to the control circuit in the pixel-specificTCC unit in each pixel, and wherein the control circuit in thepixel-specific TCC unit controls an amount of the pixel-specific firstportion of the analog charge to be transferred based on modulationprovided by the analog modulating signal.
 21. The system of claim 19,wherein the processor triggers the light source to project the laserpulse, wherein the light source is one of the following: a laser lightsource; a light source that produces light in a visible spectrum; alight source that produces light in a non-visible spectrum; amonochromatic illumination source; an Infrared (IR) laser; an X-Yaddressable light source; a point source with two-dimensional (2D)scanning capability; a sheet source with one-dimensional (1D) scanningcapability; and a diffused laser.
 22. The system of claim 19, whereinthe device in the pixel-specific TCC unit is one of the following: aPinned Photo Diode (PPD); a photogate; and a capacitor.
 23. The systemof claim 19, wherein the threshold is at least 400 μV per photoelectron.